Lasers are used as a means for bonding electrical components, such as leaded integrated circuit chips, to printed circuit boards. The laser beam is typically directed at the juncture of a component lead and a contact pad for the purpose of forming a welded, soldered, or brazed joint. In many such applications, the portion of the conductive element, such as the lead, which is impinged by the laser beam has a surface which is absorptive of the laser beam energy, causing a metallic reflow which quickly cools to form the joint.
On the other hand, in other applications it may be necessary to bond components with conductive elements which are not necessarily absorptive of the laser beam energy. For example, certain component leads or contact pads may be made out of materials such as copper, or copper or alloy 42 plated with gold. Such materials may be highly reflective of the beam of commonly used industrial lasers, which thereby makes it difficult to form a reliable physical and electrical contact between the lead and pad. Although it may be possible to cause such highly reflective surfaces to bond by concentrating the laser beam on the reflective surface for a relatively long period of time, this has a degrading effect on manufacturing efficiency and may damage the board by burning it.
One approach taken to address the problems of laser soldering a reflective surface is shown in U.S. Pat. No. 4,733,039, issued to Schnable et al, on Mar. 22, 1988. The Schnable reference discusses the combination of a light absorptive additive with a solder flux composition to improve soldering efficiency.
Solder flux, however, is typically a liquid composition which may continue to flow after it is applied. Consequently it may run in between adjoining conductive elements and onto other areas of the printed circuit board. Thus, if a laser beam is used to sweep around a component, and the light absorptive flux has spread to the areas adjoining the component, it is possible that the printed circuit board may be burned by the laser, resulting in a carbonized trace which may form an unintended electrical connection between adjoining contact pads. Additionally, if a flux containing a particulate or resulting in a particulate were to get in between the surfaces of the two elements being bonded, this could interfere with bonding or weaken the solder joint.
Another approach to the problem of bonding highly reflective surfaces is shown by U.S. Pat. No. 5,008,512, issued to Spletter et al on Apr. 16, 1991. The Spletter method for bonding two highly reflective electrical members is to coat a first electrical member with material that is absorptive of the laser energy, has a lower melting point than either the first or second electrical member, and has a low solubility in the solid alloy of the electrical members. An example of such a coating is the application of a tin coating on copper. Such an approach, however, involves the deposition of the metallic coating onto the conductive elements of the components. Common deposition techniques, such as plating and evaporative methods, tend to be costly.
What is needed therefore is a method of coating the reflective surfaces of the conductive elements of the component with a material that is absorptive of the laser beam energy. Additionally, it is desirable to have a coating material that is stable such that the coating material does not inadvertently run onto other surfaces where it may impair the reliability of the bonding process. Preferably, it should only be applied to the surface of the conductive element that is impinged by the laser. And finally, the process should be inexpensive to implement.